1. Field
The present subject matter relates generally to phase compensation in the context of gain switching of an automatic gain control, for example, as might be used in front-end processing of signals to be supplied to an Orthogonal Frequency Division Multiplex (OFDM) receiver.
2. Background
Wireless communication systems are widely deployed to provide various types of communication such as voice, data, and so on. Wireless communications may utilize a number of different multiplexing techniques. One such technique that is becoming increasingly common is Orthogonal Frequency Division Multiplexing or “OFDM.” OFDM effectively partitions an operating radio frequency (RF) band into a number of frequency sub-channels. Each sub-channel uses a respective sub-carrier. The precise frequency spacing among the sub-carriers provides the “orthogonality.” High-rate data is effectively transmitted in parallel as a set of parallel low data rate streams, on the OFDM sub-carriers. In a given transmission burst, each sub-channel transmits one data symbol. More specifically, data bits intended for transmission are grouped and encoded into symbols. During each burst, one of the coded symbols is modulated on one of the sub-carriers, e.g. using QAM or QPSK modulation. As a result, during one such burst, the N sub-carriers carry N symbols in parallel.
Front-end processing in wireless receivers, including OFDM receivers, often involves an automatic gain control (AGC) function, which effectively normalizes the level of the input signal, for example, to limit the distortion level, before demodulation. AGC also is used in a variety of other signal processing applications. Essentially, amplification or attenuation is applied to the signal, based on a comparison of signal strength to a threshold level.
Many signal processing implementations for wireless receiver circuitry or the like involve conversion from analog to digital form. In wireless receivers, for example, the demodulation and subsequent decoding are performed in digital signal processing. In such signal processing circuits, AGC typically is performed as part of the analog processing prior to the analog to digital conversion, although some implementations also provide controlled digital gain processing, e.g. by a digital variable gain amplifier (DVGA). For example, as the energy of the received radio frequency (RF) signal varies, the AGC keeps the energy seen by the A/D converter within bounds, by either attenuating or amplifying the input signal.
However, where the gain applied by the AGC across a broad spectrum band such as a band containing the sub-carriers of an OFDM communication, AGC switching or stepping of the gain between discrete states can introduce phase jump in the attendant RF down-conversion. The phase error in the signal can cause packet errors and thus decreased receiver performance. Hence, a need exists for a technique to effectively compensate for a phase jump caused by the switching of the gain of the AGC between discrete states or steps.